736 CUTTLEFISH is distinguished by the possession of numerous large-sized cells filled with pigment-granules (" chromatophores "). These pigment-cells are capable of expanding and contract ing in their dimensions, and of altering in shape, and below them are other flattened nucleated refracting cells, which co-operate with the former in the production of the mar vellous play of changing colours which the cuttle-fishes exhibit under excitement or irritation. The muscular system of the cuttle-fishes is well developed, the fibres being long and spindle-shaped, and only in certain situations (e.g., in the branchial hearts) transversely striated. The mantle is in all highly muscular, but the most important muscular organs are the " arms," the "tentacles," the "fins," and the "funnel." The "arms" are long processes produced by the splitting up of the antero-lateral margins of the foot, and the mouth is placed in the centre of their bases. In all the cuttle-fishes eight arms are present, but the so-called Decapods have in addition the " tentacles," whilst the Octopods are devoid of these supplementary processes. The arms are longer or shorter pointed processes, formed principally of longitudinal muscles, with well-marked radial bundles of fibres, and having a nerve and an artery occupying the axis of each. They are placed symmetrically round the mouth, forming a dorsal pair, a ventral pair, and two lateral pairs on each side ; and their bases are connected by an inter-brachial membrane, which in some instances (Cirrhoteuthis) extends nearly to their points. On the inner surface of the arms are placed the suckers (" acetabula "), in the form of muscular cup-like discs, which may be sessile or stalked, and which are arranged generally in one or two, or rarely in four, rows. Each acetabulum consists of a cup, the margin of which is formed by a muscular ring, sometimes strengthened by a horny girdle (which may be smooth or may be pro duced into teeth), whilst its centre is occupied by an elevated papilla composed mainly of radial muscular fibres. When the sucker is applied to any object, the contraction of the muscular fibres causes the depression of this muscular papilla, and creates a partial vacuum, thus enabling each sucker to act as a most efficient organ of prehension their action being sometimes supplemented (as in Onychoteuthis] by the conversion of the central papilla into a horny hook. The so-called " tentacles " of the Decapod Cuttle-fishes resemble the true arms in structure, but are very much longer, and only carry suckers on their swollen and club- shaped extremities. They are placed on the ventral surface of the animal, between the third and fourth pairs of arms (counting from the middle line of the back) ; they may or may not be retractile into pouches placed below the eyes ; and they may attain a length many times greater than that of the body itself. The tentacles are organs of prehension, and the arms are, in addition, employed by the animal in locomotion, enabling it to walk head downwards, at the bottom of the sea, or, when webbed, to swim through the water in a retrograde manner. One of the arms of the male cuttle-fishes, as will subsequently appear, is also more or less largely engaged in the work of reproduction, and may for this reason be greatly modified. The sides of the body in all the Decapod Cuttle-fishes, and in a few of the Octopods (e.g., Pinnoctopus), are more or less extensively produced into muscular expansions or fins, supported internally by a cartilaginous basis. These fins are employed by the animal in swimming head fore most, and they may extend along the whole length of the metasoma (as in Sepia), or they may be confined to the hinder end of the body (as in Loligo, Cheiroteuthis, Onyckotcuthis, &c.) The " funnel " of the cuttle-fishes is a muscular tube, formed by the union of the " epipodia," and placed on the lower surface of the body, with its anterior extremity pro jecting beyond the mantle, whilst it opens posteriorly into the pallial chamber. It serves for the extrusion (by means of the outgoing respiratory currents) of the undigested portions of the food and of the excretions of the kidneys and ink-sac ; whilst the water which has passed over the gills is expelled through it in a succession of jets, subserv ing in this way the secondary purpose of driving the animal backwards through the water. As regards the digestive system, the mouth is placed centrally, surrounded by the bases of the arms ; and it conducts into a powerfully muscular buccal cavity, in which are contained two strong horny jaws and a well- developed " tongue." The jaws, or " mandibles," are purely horny (not partially calcareous, as in the Pearly Nautilus and they have very much the form of a parrot s beak, working vertically, the lower one projecting most and receiving the upper mandible within it in the act of biting. The so-called " tongue " is a muscular organ, part of which is covered with numerous papillae, and is apparently an organ of taste ; whilst another portion is developed into a lingual ribbon essentially similar in its structure to the " odontophore " of the Gasteropoda. The oesophagus sometimes simple (Decapoda), sometimes provided with proventricular or crop-like dilatations (Octopoda) con ducts from the buccal chamber to the stomach, the latter organ being of large size, highly muscular, of a generally rounded shape, and having appended to its pyloric ex tremity a capacious diverticulum, into which the bile-ducts open. Into the oesophagus open the ducts of one or two pairs of salivary glands ; and the liver is of large size and highly developed, whilst certain glandular structures which pour their secretion into the bile-ducts are believed to re present the pancreas. The intestine is usually short, mostly of nearly uniform calibre, straight or slightly con voluted, and terminates in an anal aperture placed in the median line of the pallial chamber close to the base of the funnel. The excretory organs of the cuttle-fishes are the kidneys and the ink-sac; and the integumentary sinuses in connection with the so-called " aquiferous pores " may possibly also have an excretory function. The kidneys (r, r, fig. 2) are spongy, cellular, tufted, or massive organs appended to the two posterior branches of the vena cava, and sometimes developed on others of the principal veins, just before they open into the branchial hearts. They are contained, along with the veins to which they are attached and the corre sponding branchial heart on each side, in two serous sacs, which are separated centrally by the chamber containing the systemic heart, and which open by distinct apertures into the pallial chamber. The renal appendices are in direct communication with the veins on which they are situated, and have the form of membranous, often plicated sacs, covered externally with a layer of glandular cells which secrete a yellowish fluid. This fluid escapes into the serous sacs surrounding the kidneys, and is thence expelled into the mantle-cavity by the apertures before mentioned. The identity of this fluid with the renal secretion of the higher animals is shown by its containing uric acid (as proved by Harless). The ink-sac is a glandular organ, present in all known Dibranchiates, generally of a pyriform shape, situated in different portions of the visceral chamber, but communicat ing by a longer or shorter duct either with the terminal portion of the intestine, or, more commonly, directly with the pallial cavity by a special aperture of its own, situated close beside the anus and at the base of the funnel. The ink-sac has strong fibrous walls, often with a silvery lustre, and its secretion is a brown or black fluid, containing a large amount of a carbonaceous pigment (" sepia "), along
with various mineral salts. It is employed by the cuttle-